Foraging Strategies

Animal Sciences
COPYRIGHT 2002 The Gale Group Inc.

Foraging Strategies

The term "forage" means to wander in search of food. Every animal has a particular method of locating food, whether they smell it, find it by sight, or detect it by chemical means. Animals seek out food both individually and in groups.

Collecting food as efficiently as possible allows a species to propagate its genes more effectively. The optimal foraging theory, developed by Robert H. MacArthur and E. R. Pianka, states that food gatherers that do a better job of increasing the benefits and of decreasing the costs of foraging should procreate more effectively than those whose feeding activities yield lower net benefits. Foraging decisions are, in effect, cost-benefit problems that animals have to solve.

Food, whether animal or vegetable, provides stimuli that predators can detect. Waste products of animals give off olfactory signals that help predators locate their next meal. For example, when dung-eating beetles smell far-off feces, they quickly take to the air and forge their way along the odor trail leading straight to their primary food source. A more highly specialized adaptation involves the urine and dung of the vole, a small rodent, which does more than release olfactory signals. The waste products of the vole reflect a certain amount of ultraviolet radiation that is invisible to the
human eye but is clearly evident to the kestrel, a small hawk. From the air the kestrel can detect the ultraviolet markings left by the vole, which increases the kestrel's hunting success.

Foragers do not always work alone. Sometimes their companions inform them of food locations. Social insects such as bees, wasps, ants, and termites have evolved incredible techniques for transferring this kind of information. Probably the most fascinating is the complex dance of the honeybee. It is performed when a forager has found pollen or nectar and has returned to the hive. Depending on the distance and direction of the food source in relation to the sun, the honeybee will perform either a waggle dance or a round dance. The round dance informs other worker bees that a food site is located within fifty meters of the hive. The waggle dance, depending on the number of abdomen waggles and direction faced when waggling, illustrates both the distance and the direction of a food source in relation to the hive. Karl von Frisch spent twenty years experimenting with bees that he had trained to visit particular feeding stations. By studying their dances he determined that their behavior changed significantly depending on the distance and direction of a food source's location.

What about solitary animals who go unaided in the search for prey? Some rely on deceitful measures to catch their dinner. One ingenious method is used by the bolas spider (Mastophora dizzydeani ), which releases a scent identical to the sex pheromone of certain female moths. When the male moth of the species goes in search of a mate, he may encounter instead a bolas spider armed with a sticky globule attached to a long, silken thread. The spider throws the blob, hits the moth, and then feasts on the captured prey. By employing the deceptive scent, the bolas spider lures insects within attack range. This maximizes the spider's success rate while reducing its energy output.

Because of potentially toxic foods, predators must pay attention to warning coloration and behavior of fauna prey and carefully avoid more difficult-to-detect toxins in flora. Herbivores must be able to determine whether the
plant they have selected to eat has low concentrations of toxic terpenoids— poisons that many plants incorporate into their tissues to repel consumers.

A study of the herbivorous Costa Rican howler monkeys illustrates how some animals deal with this problem. Foraging very carefully, these monkeys avoid toxic leaves and those low in nutritional value. Although the choices they make in selecting particular leaves may raise the costs of foraging, they ingest fewer poisons and more usable proteins. For example, howlers tend to avoid foraging in common tree species, opting instead to eat the leaves from scarcer species. These preferred tree species, it turns out, have lower levels of alkaloids and tannins. Alkaloids are poisonous to howlers, and tannins make leaves harder to digest. Also, the monkeys tend to feed on only the petiole—the leaf part lowest in toxins—while discarding the more toxic leaf blade.

The optimality theory does not always work in a clear-cut fashion. Some animals must balance the cost of consuming more food per unit of foraging time against the risk of becoming food themselves. For instance, the hoary marmot, a relative of the groundhog, stays close to its burrow on rocky slopes and feeds primarily on the heavily grazed margin of meadow located nearby rather than venturing further into greener pastures. This reduces its risk of becoming prey for an eagle or coyote. Thus, it spends more time looking out for food than for danger. Although this may compromise foraging efficiency in caloric or nutritional terms, it keeps them alive. Whirligig beetles behave in much the same way, restricting their movements to the dense cluster of beetles that form on the water's surface. In this way, they steer clear of being eaten yet sacrifice foraging opportunities that exist beyond the safety of numbers.

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